ATM is a switching and multiplexing technique designed for transmitting digital information, such as data, video, and voice, at high speed, with low delay, over a telecommunications network. The ATM network includes a number of switching nodes coupled through communication links. In the ATM network, bandwidth capacity is allocated to fixed-sized units named “cells.” The communication links transport the cells from a switching node to another. These communication links can support many virtual connections, also named channels, between the switching nodes. The virtual connections, for example a Virtual Channel Connection (VCC) or a Permanent Virtual Circuit (PVC), assure the flow and delivery of information contained in the cells.
Each cell contains a cell header and cell data. The cell header includes information necessary to identify the destination of that cell. The components of the cell header include, among other things, a Virtual Channel Identifier (VCI) and a Virtual Path Identifier (VPI), for collectively identifying an ATM connection for that particular cell, and a Payload Type Identifier (PTI), for indicating whether the cell is sent from one user to another, whether cell data refers to administration or management traffic, and whether congestion is present within the network.
The ATM Forum, which is a user and vendor group establishing ATM standards, has also defined several ATM service categories, used in characterization of a virtual connection. For example, among them are categories such as (1) a Constant Bit Rate (CBR), which supports a constant or guaranteed rate to transport services, such as video or voice, as well as circuit emulation, which requires rigorous timing control and performance parameters; (2) a Variable Bit Rate (VBR), real time and non real time, which supports variable bit rate data traffic with average and peak traffic parameters; (3) an Available Bit Rate (ABR), which supports feedback to control the source rate in response to changed characteristics in the network; and (4) an Unspecified Bit Rate (UBR).
FIG. 1 illustrates a prior art ATM network 100, typically including several network nodes, also known as switching nodes, 110 connected through single communication links 120. The network 100 is a data transmission network with guaranteed bandwidth and quality of service. Typically, end users 130 access the network 100 and connect to the nodes 110 via similar links 120. Generally, the illustrated communication links 120 carry traffic from many sources to many destinations and may support multiple virtual connections. Although these virtual connections may be statistically multiplexed onto the same link, the network 100 must still meet certain quality of service requirements for each connection. Therefore, the network must allocate an appropriate bandwidth to each connection without disturbing the other connections already in traffic.
Attempts made to allocate the appropriate bandwidth to each connection have resulted in a Connection Admission Control (CAC) process, which uses source traffic characteristics, such as peak cell rate (PCR), average cell rate (ACR), burstiness, and peak duration, and the required quality of service parameters, such as cell loss ratio, cell transfer delay, and jitter, to make a decision whether to accept or reject the connection and to access the amount of bandwidth required by the connection. In the network 100, a user 130 and the network negotiate a traffic contract, whereby the user supplies the traffic characteristics, and the desired quality of service, and the network performs a CAC process to determine whether there is enough free bandwidth to accept the connection.
If the network carries voice communications, such as voice calls, and certain cells are dropped, then all voice calls will suffer degraded voice quality.